Gary Lineker was at the launch of the Spell & Go campaign by Walkers in central London earlier this year

An ad for a Walkers Crisps competition offering 20,000 holidays has been banned after more than 100 consumers complained the trips were virtually impossible to win.

The Spell and Go promotion, presented by BBC Match Of The Day host Gary Lineker, invited participants to collect letters by posting codes from packs of crisps to spell out the names of one of 26 possible destinations for the chance to win a holiday.

But customers took to social media to complain that it seemed impossible to get the letters C, D or K to complete the names of some of the places.

Some 112 people complained to the Advertising Standards Authority (ASA) that the competition, advertised in May on the Walkers website, Twitter account and Facebook page as well as on product packaging and on television, withheld certain letters which were needed to spell out the destinations, with some reporting problems relating to the website accepting the codes.

Defending the promotion, Walkers said all 26 destinations included at least one of the letters C, D or K - type 1 letters - and the company had ensured enough of these letters were in circulation to allow for 20,000 holidays to be won.

As far as it was aware, all consumers who experienced problems with codes being accepted by the website had had the issue resolved.

Following the complaints, the ASA said it was satisfied a small proportion of the total number of letters in circulation were type 1 letters, rather than these letters being withheld.

However the complaints watchdog was not satisfied with another part of the competition, the "random swap" function, which allowed participants to swap letters within a "pool" on the competition's website, which stated "all letters are treated equally".

The ASA discovered the pool was only made up of type 2 letters, meaning C, D or K could not be won.

The ASA said it "considered the original limitation of the random swaps mechanism to only type 2 letters was a significant condition likely to influence a consumers' decision" and the omission was "misleading and likely to cause unnecessary disappointment to consumers".

It told Walkers that in similar future promotions, it must ensure "significant conditions for all aspects of the promotion were communicated to consumers".

A Walkers spokeswoman said: " We welcome the ASA's recognition that our Spell and Go promotion was fair as everyone who participated had an equal chance of winning one of the 20,000 holidays available.

"We appreciate that the online letter swapping mechanic could have been clearer and we will ensure all future promotions take this feedback on board."

She added: "Our Spell and Go promotion has been very popular and we've given 796 families four-star, seven-night holidays worth over £1.35 million.

"We're aware some customers are disappointed that they haven't been successful in winning a holiday. 20,000 holidays could have been won if all the promotional packs in the market had been played and we would have honoured all of those should that have been the case."